Optical-based sensing has several major advantages over electronic sensing because optical sensing reveals spectral fingerprints of chemical compounds rapidly and accurately, thus significantly simplifying the detection process and reducing false alarms. One of the most promising optical sensing techniques is surface enhanced Raman spectroscopy (SERS), which employs noble metal nanostructures to dramatically enhance Raman signals. With the aid of metallic nanostructures, such as gold- or silver-based nanosubstrates, a Raman signal can be enhanced by 104 to 108 times or even higher. This enhancement is due to the generation of spatially localized surface Plasmon resonance (SPR) “hot spots” where huge local enhancements of electromagnetic field are obtained. The location of “hot spots” on the metallic structures depends on the geometry of the nanostructures, the excitation wavelength, and polarization of the optical fields. SERS can potentially reach the limit of detection down to the low parts-per-billion (ppb) and theoretically to the single molecule level. Thus, SERS has been increasingly used as a signal transduction mechanism in biological and chemical sensing.
One of the most critical components for surface enhanced Raman spectroscopy (SERS) is the development of suitable substrates that can activate surface plasmon resonance (SPR). In principle, sharp edges of the metal surface topography can produce SPR as induced by an incident excitation laser, thus generating an enormously enhanced electromagnetic field of signals that occur within highly localized optical fields around the metallic structures. When designing a surface structure suitable for SERS application, the size of the metal islands, grains, or particles constructed onto the supporting substrate varies from several nanometers to microns. Generally, a nanoscale structure has multiple advantages over a microscale one because the plasmon localization becomes more intensified at a nanoscale due to a strong spatial confinement effect. As the size of bodies decrease, their surface-to-volume aspect ratios increase. A high surface-to-volume ratio gives rise to an increased number of probe molecules available for capture in the vicinity of metal surface within a distance on the order of nanometers.
Current efforts for nanostructure development can be categorized as either direct or indirect methods. Direct methods involve manipulating the metal directly to prepare a metal substrate with preferred micro- or nanostructures, while indirect methods employ other materials, such as ceramics, to prepare the preferred micro- or nanostructures first and then incorporate the metal onto these structures.
However, there are technical and non-technical challenges in fabrication of SERS substrates that significantly impede the commercial applications of SERS. For example, most existing SERS substrates exhibit inconsistent activities and it is a common problem that a subtle change in the substrate manufacturing process can produce significant changes of the Raman signal. Such inconsistencies make quantitative or even semi-quantitative analysis difficult.
Therefore, there is a need for better SERS substrates with well-controlled surface characteristics to achieve measurement accuracy and consistency in SERS analysis. There is also a need to provide new and improved fabrication method for manufacturing SERS substrates with well-controlled surface characteristics. There is yet another need to develop new and improved SERS analysis protocols for various applications, such as food safety, water safety, homeland security and other areas. The present novel technology addresses these needs.